Additive Manufacturing of Hot-Work Tool Steel by In Situ Alloying Using Laser Powder Bed Fusion and Directed Energy Deposition—Strategies to Improve Chemical Homogeneity

Abstract

The production of complexly shaped tools with integrated cooling channels by additive manufacturing represents a promising approach to minimize lead times and the material footprint in tool making. Unfortunately, typical carbon-martensitic steels suffer from cracking during additive manufacturing. To prevent cracking, adjustments in alloy design can be made. Therefore, modification of available powders by in situ alloying is promising. Thereby, an alloy is formed from a mixture of powders. This, however, can promote chemical inhomogeneous parts. This work investigates the possibility of improving the chemical homogenization of a powder mixture. As tracers, high-melting raw materials, ferrotungsten and ferromolybdenum, are included, which have proven challenging to be homogenized using additive manufacturing. Now, the effect of double laser exposure in laser powder bed fusion and of enlarged melt pools in directed energy deposition on the homogenization are analyzed. The chemical homogeneity is assessed statistically based on 2D-energy dispersive X-ray spectrometry data. Furthermore, the local hardness is related to the chemical homogeneity. A guideline is given to which extent chemical inhomogeneities resulting from in situ alloying can be tolerated regarding mechanical properties and defect formation. The findings indicate that applying remelting in laser powder bed fusion is promising to produce parts of high geometric accuracy from powder mixtures.